The novelist told Baroness Thatcher the product "takes oxygen to every part of the body, including the brain" and that as someone aged nearly 88 herself, she found it "fantastic".

And just a few weeks later, on 3 July 1989, Dame Barbara sent another product to the prime minister's diary secretary, Amanda Ponsonby, for her to give Mrs Thatcher to take on a long trip to the Far East. She said it would not induce sleep, but "just stops that awful feeling in the head and ears".

In November of the same year, Baroness Thatcher wrote to the novelist after a "wonderful luncheon" and thanked her for a "golden acorn".

It is not clear what this item was, but Baroness Thatcher wrote: "I shall need it in the coming days!"

At the time, her chancellor had resigned the month before, and a leadership challenge had been launched.

More from Margaret Thatcher's archives:

Elsewhere in the newly released files are references to Baroness Thatcher taking royal jelly and vitamin C.

According to a profile in Vanity Fair magazine in June 1989, the former prime minister was also fond of "electric baths" in which 0.3 amps of electricity was run through water in a bid to stay youthful.

Chris Collins, of the Margaret Thatcher Foundation, said he believed her interest in alternative medicines was genuine, though references in the archives were "obscure, perhaps deliberately so".

The "electric baths" report generated a lot of attention in the world's media. He said Baroness Thatcher's press secretary Bernard Ingham could not treat lightly "the impression of dottiness, of a woman 'slightly off her trolley'."

Cameron's Europe briefing

The files, available for the public to view from Monday at www.margaretthatcher.org, also include a briefing paper on the benefits of the coming European Single Market, written by a young David Cameron, who was then at the Conservative Research Department.

"We believe that creating a single community market will be good for Britain, good for Europe and good for the world," runs the opening quote, from the party's manifesto for the European elections in 1989.

Pages initialled by "DC" set out how the single market, due to start in 1992, would help business, workers, consumers, savers, holidaymakers. It detailed the changes in different sectors of the economy and was extremely positive about the effect this would have.

The files also reveal that Mrs Thatcher tried to refuse the resignation of her chancellor Nigel Lawson in October 1989, calling it "an absurd proposition" and a "flimsy and unworthy proposal". According to the note, she told him to "go away and think again".

Documents also refer to a leadership challenge launched by Sir Anthony Meyer later that year.

While Baroness Thatcher won the MPs' vote decisively that December, documents show at least one of her advisers feared this was "the beginning of the end of the Thatcher era". She left office in 1990, after 11 years as prime minister.